1. Field of the Invention
The present invention relates to a fuel cell, and more particularly to a method for operating a direct methanol fuel cell in which an aqueous methanol solution is supplied directly to the fuel electrode of the fuel cell main body, and a fuel cell system.
2. Description of the Related Art
In recent years, increased awareness of environmental issues and energy concerns have prompted considerable interest in fuel cells, and fuel cells are already in use as the driving power supply for some vehicles and as domestic cogeneration systems.
Furthermore, direct methanol fuel cells, which can be reduced in size due to the fact that the methanol fuel is supplied directly to the fuel electrode in the form of an aqueous methanol solution, with no requirement for reforming or gasification, have also been proposed as potential power supplies for small, portable electronic equipment (see Japanese Patent Laid-Open Publication No. 2003-132924).
An example of a conventional fuel cell system of this type is shown in FIG. 8. In FIG. 8, the fuel cell system includes a fuel cell main body 31, a dilution tank 32, a fuel supply pump 33, an air pump 34, a gas-liquid separator 35, a return pump 36, a methanol tank 37, a methanol pump 38, and a control circuit 39. The fuel cell main body 31 includes a fuel electrode (a negative electrode) to which an aqueous methanol solution is supplied on one side of an electrolyte film formed from a proton conductive polymer film, and an air electrode (a positive electrode) to which air containing oxygen is supplied on the other side of the electrolyte film. The dilution tank 32 stores the methanol as an aqueous methanol solution with a concentration of 1 to 20%, for example 5%. The fuel supply pump 33 supplies the aqueous methanol solution from the dilution tank 32 to the fuel electrode of the fuel cell main body 31. The air pump 34 supplies air to the air electrode of the fuel cell main body 31. The gas-liquid separator 35 separates carbon dioxide, and gasified methanol and water from the discharge fluids discharged from the fuel electrode and the air electrode and then discharges them externally. The return pump 36 returns generated water and unused methanol to the dilution tank 32. The methanol tank 37 stores the raw material methanol at a concentration of several tens to 100%. The methanol pump 38 supplies methanol from the methanol tank 37 to the dilution tank 32 so as to achieve a predetermined concentration within the dilution tank 32. The control circuit 39 controls each of the above pumps 33, 34, 36, and 38. A large quantity of the aqueous methanol solution is supplied to the fuel electrode of the fuel cell main body 31, in order to prevent reductions in the concentration of the aqueous methanol solution in the vicinity of the discharge port of the fuel electrode, and the unused methanol is then reused.
Furthermore, in addition to the above type of closed-type fuel cell system, fuel cell systems have also been known in which a recovery and storage section is incorporated within the fuel tank section used for housing the fuel, and specific products such as water are separated and recovered from the discharge fluid from the fuel cell main body and then stored in this recovery and storage section in order to prevent these products from being discharged outside the fuel cell and causing damage to neighboring equipment (see Japanese Patent Laid-Open Publication No. 2002-216832).
However, in the closed-type fuel cell system shown in FIG. 8, because a large proportion of the methanol supplied to the fuel electrode of the fuel cell main body 31 remains as unused methanol, which must be recycled from the discharge port of the fuel electrode and passed through the gas-liquid separator 35, completely preventing the external discharge of methanol that has vaporized in the gas-liquid separator 35 is impossible, and some harmful methanol is discharged externally.
What is worse, a portion of the methanol supplied to the fuel electrode penetrates the electrolyte film (cross leak) and is burnt at the air electrode. Consequently, in these systems the combined quantity of methanol either discharged externally from the gas-liquid separator 35 or cross leaked and subsequently burnt is considerable, meaning the fuel efficiency is poor.
According to tests using a specific closed-type fuel cell system, when a 5 wt % aqueous methanol solution was supplied at a flow rate of 2 cc/minute, and a power level of 0.5 to 20 W was output, the concentration of the aqueous methanol solution at the fuel electrode discharge port fell to a value of 3 to 5 wt %. If the quantity of methanol supplied in this state is deemed 100%, then the quantity of methanol vaporized and discharged with the exhaust gas is 28%, and the quantity of methanol that undergoes cross leak reaches 36%, meaning only 36% of the methanol is actually contributing to power generation. Of this 36% contributing to power generation, 24% is consumed as heat generation due to resistance loss during reaction, and a mere 12% is converted to electrical energy.
If the fuel cell system with a recovery and storage section disclosed in Japanese Patent Laid-Open Publication No. 2002-216832 is applied to a direct methanol fuel cell, then if a recovery and storage section with a simple, compact construction is employed, preventing external discharge of vaporized methanol during the separation and recovery of the methanol discharged from the fuel electrode is difficult. However, if an attempt is made to recover and store all of the discharged fluid except for water, then a large quantity of fluid must be stored in the recovery and storage section, meaning a compact construction is impossible.